Passive Nuclear Reactor Emergency Cooling System Using Compressed Gas Energy and Coolant Storage Outside Nuclear Plant

ABSTRACT

A passive safety system for a nuclear power plant ( 100 ) cools a nuclear power plant after shutdown (SCRAM) even when all primary water circulation has been disabled. The system comprises a source of compressed gas ( 112, 805 ) that can be its only source of operating energy, a source of water ( 106, 500 ), and a plurality of plumbing components. The system is located nearby but outside of the plant where it will not be damaged in the event of an accident inside the plant. In one embodiment, the system is located underground. In another embodiment, the system is portable so that the gas and water are carried in tanks ( 500, 510 ) on railroad cars or other wheeled conveyances. The portable system is located above ground, or optionally in a covered trench ( 705 ). In an alternative embodiment, only compressed gas is used to cool the plant.

CROSS-REFERENCE TO RELATED CASE

This application claims priority of applicant's copending provisionalpatent application Ser. No. 61/930,369, filed 2014 Jan. 22.

BACKGROUND Prior-Art Emergency Cooling Systems

A nuclear power plant generates electricity by using a nuclear reactorto boil feed water to create high-pressure steam. This steam in turndrives turbines, which in turn drive electrical generators that createthe electricity. The feed water is called the “primary coolant” in thenuclear industry since it absorbs and carries away the heat generated bythe nuclear fuel rods in a reactor pressure vessel (RPV) inside thereactor. After the steam drives the turbines it is condensed back towater and is recirculated back to the RPV to be heated again tocontinuously generate new steam for the turbines.

Sometimes it is necessary to shut down a nuclear plant for operationalreasons or because of an accident or terrorist attack, loss of outsideelectrical power, or a natural event such as an earthquake or fire. Thussome means must be provided to stop the nuclear chain reaction which iscontinuously occurring in the reactor. This is done by inserting controlrods and-or chemical substances into the reactor to absorb the fissionneutrons that drive the nuclear chain reaction and heat the primarycoolant. However, stopping the nuclear reaction does not eliminate thedanger or cool the reactor adequately. A major source of heat, calledthe “decay heat,” remains. This decay heat is generated by radioactivedecay of isotopes within the nuclear fuel rods. Even after shutdown thisheat must be continually removed from the nuclear fuel rods for days orweeks in order to stabilize the reactor. Otherwise, a nuclear “meltdown”can occur, as happened in the Three Mile Island, Pa. (1979) andFukushima Daiichi, Japan (2011) nuclear accidents.

During normal power generation the primary coolant circulates throughplumbing (termed the “primary boundary”) coupled to the RPV. The primaryboundary includes an outlet pipe from the RPV (commonly called the “hotleg”) that carries the steam generated from the primary coolant to theturbines. After condensation back to water the primary coolant isreturned to the RPV by a pipe called the “cold leg.”

Existing Emergency Core Cooling Systems (ECCS) in water-cooled reactorsall depend on continued circulation of the primary coolant around thefuel rods to absorb and dissipate the decay heat. The cooling fluid iscirculated by large electrical or steam driven pumps that require backuppower sources in case the nuclear plant is disconnected from theelectrical grid that it serves. At least one newer and approved ECCSdesign, the AP1000 (see Non-Patent Literature below), called a “passivesafety system,” does not require backup emergency power. The AP1000utilizes additional water from a large gravity flow storage tank mountedabove the reactor to cool the primary coolant circulating through thereactor. The AP1000 relies on continuing decay heat to circulate theprimary coolant through a heat exchanger by convection. Hence, theAP1000 design by itself cannot bring a reactor to “cold shutdowncondition” (less than 100 degrees C.), as noted below and in thediscussion of the cited patent to Sato, infra.

Naval nuclear reactors, i.e., those used in submarines and ships, are ofless concern because they are surrounded at all times by abundant oceanwater to cool the fuel rods in a reactor in an emergency. This degree ofsafety has not been available for land-based nuclear plants.

The Fukushima nuclear disaster presented a horrible reality to theworld. It showed that all existing emergency reactor cooling equipmentin the world's nuclear power plants today can be disabled by the forcesof nature or by terrorists. The existing emergency cooling systems arefragile in many ways. The biggest weakness is that the essentialemergency cooling apparatus in most nuclear plants is co-localized inthe nuclear plant buildings. It is rather disturbing that after theFukushima accident there were several nuclear plant shutdowns in theU.S. where some of the backup electrical generators failed and alast-ditch battery backup system had to be used.

Water Flow Urged by Gravity.

An example of water delivery by gravity flow is found in theabove-mentioned AP1000 ECCS nuclear power plant design by WestinghouseCorp. of Cranberry Township, Pa., USA. This design employs a large tankof emergency cooling water that is placed atop a structure within anuclear plant. When released in an emergency, gravity causes thissecondary cooling water to flow through a heat exchanger where itextracts the decay heat from the hot primary coolant in the reactor. Solong as the primary coolant circulation system is intact and operating,the secondary cooling water delivered by gravity is turned into steam.The steam is condensed by cooling by outside air flow over surfacesattached to the containment building. In this design, the gravity watercirculation equipment is all inside the nuclear plant structure. Itslarge emergency water tank is mounted high above the reactor pressurevessel and thus can be disabled by severe earthquakes. Any assault on anuclear plant that damages the internal plumbing in which the primary orgravity cooling water are circulating can disable the AP1000 ECCS. Adedicated terrorist attack that damages a few critical pipelines andvalves inside the nuclear plant can disable all emergency coolingoperations. This can lead to a disastrous meltdown of the fuel rods.

Water Flow Urged by Compressed Gas.

Domestic water tanks that are pressurized by compressed air above thewater are well known. These tanks are designed to provide water in agiven pressure range without requiring a water source to be energizedfor each delivery of water into or out of the tank. These tanksgenerally employ an impermeable membrane between the compressed air andthe water below in order to avoid loss of the air by absorption of theair into the water (aeration).

Gas pressurized water tanks called “accumulators” are used in nuclearplants today. The water is commonly loaded with a borate solution orother “neutron poisons” that stop the fission reaction in a nuclearreactor during a nuclear emergency. The tanks maintain a high pressurenitrogen or water vapor bubble above the water. Because theseaccumulator tanks are placed inside the nuclear plant building and areunder constant high pressure, their size is limited. They do not storesufficient cooling fluid to absorb the decay heat for many hours, letalone many days, from a nuclear reactor immediately after shutdown.

Very high static gas pressures (up to 200 bar) cannot be used in thewater tanks above unless the tanks are relatively small. If the tankdimensions are large or the gas pressures too high, the cannot withstandthe high hydraulic forces on the walls of the tank without bursting.

Prior-Art References

The following is a list of some possibly relevant prior art that showsprior-art emergency cooling systems for nuclear power plants. Followingthis list I provide a discussion of these references.

U. S. Utility Patents

Patent or Pub. Nr. Kind Code Issue or Pub. Date Patentee or Applicant5,085,825 B1 1992 Feb. 4 Gluntz et al. 7,873,136 B2 2011 Jan. 18 Meseth8,045,671 B2 2011 Oct. 25 Meseth 8,559,583 B1 2013 Oct. 15 Sato

Non-Patent Literature

-   1. AMERICAN NUCLEAR SOCIETY, Special Committee on Fukushima,    Fukushima Accident 2011, March 2012, http://fukushima.ans.org/.-   2. HANSEN, JAMES ET AL., Climate Change Experts Endorse Nuclear    Power, World Nuclear News, Nov. 4, 2013,    http://www.world-nuclear-news.org/EE-Nuclear-essential-for-climate-stability-0411137.html.-   3. CAeS Compressed Air Energy Systems,    http://en.wikipedia.org/wiki/Compressed_air_energy_storage, Huntorf,    Germany 290 MW CAeS 1978, McIntosh, Ala. 110 MW CAeS 1991.-   4. WESTINGHOUSE ELECTRIC CO, LLC, The AP1000 Nuclear Plant Design,    http://ap1000.westinghousenuclear.com/index.html, Cranberry    Township, Pa., USA.-   5. U.S. NUCLEAR REGULATORY COMMISSION, AP1000 Safety Evaluation    Report, NUREG-1793, September, 2011    (http://www.nrc.gov/reactors/new-reactors/design-cert/ap1000.html)

Gluntz and Meseth both show “accumulator” tanks containing water and awell-known neutron absorbing or “poisoning” material to halt or slow anuclear reaction. The primary purpose of the accumulators is to inject aneutron poison into a reactor vessel. This rapidly shuts down thefission reaction within the reactor in case the control rods do notfunction properly, or minimizes the fission reactions after a rapidshutdown of the reactor. The accumulator tanks are limited in size andinternal pressure by their inherent bursting strength. Because of thissize limitation, the amount of water stored in an accumulator tank istoo small to perform any major cooling of a reactor. The operation of anaccumulator tank is dependent on infrastructure and piping inside thenuclear plant and, as such, it will be disabled if critical elements ofthe infrastructure are disabled. In addition, these tanks cannot deliverenough cooling fluid to cool a reactor at peak decay heat production formore than a few minutes or hours.

Sato describes the operational features of the above-mentionedWestinghouse AP1000 passive ECCS system. Then he describes animprovement over the AP1000. He shows a Depressurization and CoolingSystem (DPCS) that is attached to the AP1000 design to cool the reactorto “cold shutdown condition.” The DPCS uses compressed gas to propelwater out of an auxiliary water tank. Sato also shows that the watertank for his DPCS can be outside the reactor Primary Containment Vessel(PCV) and optionally buried underground. However, this DPCS apparatus isconnected to and part of the internal plumbing in a nuclear plant Likethe AP1000, Sato's DPCS system requires primary coolant to be circulatedthrough the reactor and associated plumbing attached to the reactor tocool the heated coolant and hence the reactor.

Many of the pressurized accumulator tanks used in current nuclear plantsuse heating elements to generate water vapor, which is used to provide apressurizing fluid above the stored water. This is preferable to usingstored compressed gas at high pressure inside a nuclear plant because aruptured gas tank could present a significant safety issue. However, theelectrical power required for such heating elements may not be availablein a disabled nuclear power station.

While each of the above systems may be suited for their particular use,all have one or more deficiencies as noted.

SUMMARY

The emergency reactor core cooling system (ECCS) system and methoddescribed herein overcomes one or more of the deficiencies of prior-artsystems while satisfying many requirements to prevent more nuclearaccidents under the worst conditions such as happened at Fukushima in2011. All embodiments are designed to absorb the decay heat from a largereactor after an emergency shutdown (called “SCRAM” in the industry for“Safety Control Rod Axe Man”) when all internal ECCS systems have beendisabled and none of the existing and prior art ECCS systems, passive orotherwise, can function. Some embodiments assume only that the reactorpressure vessel RPV can still receive and hold externally injectedcooling fluid long enough to cool the nuclear fuel rods therein. Anotherembodiment cools a reactor under the very worst condition that the RPVis so damaged (ruptured) that it can no longer hold water around thefuel rods therein.

All embodiments are “passive safety systems” in that the only energysource needed or used for the reactor cooling operation performed is thepotential energy stored in the compressed gas contained in the system.Various embodiments are installed outside nuclear plant buildings. Thecooling fluid used is cooling water stored separately outside a nuclearplant and-or the compressed gas itself. The various embodiments can beinstalled immediately using existing proven structural components andtechnology from the oil and gas well drilling and the railroadindustries.

The system can be installed underground or safely above ground, outsidethe structures housing a nuclear power station so that this ECCS systemis far less susceptible to damage by acts of nature or terrorism. Itdoes not depend on or need any of the existing ECCS systems oroperational cooling systems inside a nuclear plant or any emergencybackup power systems. This system requires only a connection to theexternal cooling water input port(s) that exist on all nuclear reactorsas a last resort means to cool a reactor when all internal coolingfunctions are inadequate.

One embodiment, called the Compressed Gas Emergency Cooling System(CGES) employs a gas, such as air or nitrogen, that is safely stored athigh pressure in a first tank and is used to pressurize cooling water inanother, larger tank. Unlike other gas-over-water pressure tank schemes,the cooling water tank in this embodiment is pressurized only when it isneeded during a nuclear emergency. This allows cooling water to beretained long-term in much less expensive water storage tanks. Thepresent system can bring a reactor to “cold condition” with the reactorcoolant held below 100° C. long-term. All embodiments of the presentsystem do not need nor use continued primary coolant circulation in aSCRAMMED reactor.

Another embodiment uses a compressed air energy storage system (CAeS) toprovide emergency reactor cooling by direct injection of largequantities of compressed gas. It is called NCAP for combined Nuclear andCompressed Air Power plant. The NCAP embodiment cools the fuel rods in areactor under the worst case that the RPV is so damaged (ruptured) thatit cannot hold liquid coolant. This is something that cannot be done byany other existing or approved ECCS system for water cooled nuclearreactors. The NCAP also combines a CAeS with a nuclear power station ina unique manner that achieves great reductions in operating cost andproduction of climate warming CO₂ during normal power generatingoperations. These reductions in cost and pollution more than pay for theaddition of a CAeS plant to a nuclear plant. The NCAP embodiment is anextension of the CGES design concept. The NCAP is very appropriate fornew nuclear power stations.

The CGES and NCAP systems are able to cool a large nuclear reactor formany days after shutdown when all in-plant, prior art emergency coolingsystems are disabled. The CGES and NCAP systems do not require anyoutside emergency power for their operation. They are low in cost,easily installed, and safe. As described, they can be installedimmediately using existing proven structural components and technologyfrom the oil-gas well drilling industry and the railroad industry.

ADVANTAGES

Various aspects the present systems have one or more of the followingadvantages: They are passive safety systems that require no outsidepower and hence can be more reliable. They are or can be separate fromand located away from the nuclear plant structures that contain thenuclear reactor and the in-plant emergency reactor cooling systems. Theydo not require external or backup power sources, other than their ownstored energy. They can cool a nuclear reactor for several days aftershutdown (SCRAM) when all existing in-plant cooling equipment, coolingfluid, emergency power systems are lost or disabled. They provide anadd-on system that does not require major interruption of the operationof a nuclear power plant. They can be constructed with readily availableand inexpensive standard components and they can be installedimmediately at our aging nuclear plants. Further advantages of variousaspects will be apparent from the ensuing description and accompanyingdrawings.

DRAWINGS

FIG. 1 shows a schematic diagram of one aspect of a CGES system.

FIG. 2 is a cross-sectional view of one aspect of a CGES system.

FIG. 3 is a schematic diagram of a nuclear power plant showing fluidflow after a SCRAM.

FIG. 4 is a schematic plan view showing a plurality of CGES unitsconnected to a nuclear plant.

FIGS. 5 to 7 show aspects of an embodiment of a CGES using standardrailroad tank cars for water storage.

FIG. 8 is a schematic diagram showing a NCAP system.

FIGS. 9 to 11 show CG paths inside a nuclear reactor after a SCRAM.

REFERENCE NUMERALS

100 Nuclear power plant 102 Reactor pressure vessel (RPV) 104 Primarycontainment vessel 106 Tank (PCV) 108 Water 110 Pressure 112 Tank 114Pressure 116 Heat exchanger 118 Ground level 120 Pipe 122 Valve 124 Pipe126 Valve 128 Expansion valve 130 Coil 132 Volume 134 Pipe 136 Pipe 138Pipe 140 Pipe 142 Valve 144 Spent-fuel pool 146 Pipe 148 Valve 200Casing 205 Grout 210 Plug 215 Lid 220 Gasket 225 Blocks 230 Bolts 235Brackets 240 Welds 245 Pipe 255 Valve 260 Handle 265 Pipe 270 Valve 290Pipe 295 Pipe 300 Manifold 305 Valve 310 Valve 315 Pipe 320 Pipe 325Valve 330 Valve 335 Valve 350 Manifold 400 CGES units 405 Pipes 410Valves 415 Tanks 420 Tank 425 Valve 500 Railroad tank car 505 Track 510Tanks 515 Manifold 520 Pipes 600 Railroad flat car 700 Housing 705Trench 710 Fence 715 Roof 720 Tracks 800 CAeS system 805 Cavern 810 Heatexchanger 812 Coils 814 Pipe 815 Turbine 820 Generator 822 Conductors825 Compressor 830 Pipe 835 Valve 840 Turbine 845 Generator 850Conductors 855 Conductors 870 Valve 875 Pipe 881 Pipe 900 Fuel rods 905Baffle 910 Manifold 915 Openings 920 Pool 925 Pipe 930 Valve 935 Pipe940 Valve 945 Pipe 950 Valve 1100 Plate 1105 Plate

ABBREVIATIONS

-   CG Compressed Gas-   CAeS Compressed Air Energy System-   CGES Compressed Gas over Water Emergency Reactor Cooling System-   DPCS Prior-Art Depressurization and Cooling System-   GJ GigaJoules-   MW MegaWatts-   MJ MegaJoules-   NCAP A combined Nuclear and CAeS Power plant design-   PCV Primary Containment Vessel-   RPV Reactor Pressure Vessel-   RRTC Railroad Tank Car-   SCRAM Safety Control Rod Axe Man—an emergency shutdown of an    operating nuclear power plant.-   SFP Spent Fuel Pool

OVERVIEW

In one embodiment, CG is safely stored at high pressure in one tank andis used to pressurize cooling water in another, larger tank. The CG isadmitted to pressurize the cooling water when it is needed during anuclear emergency. In one aspect the heat required for rapid expansionof large volumes of the CG is extracted from the water that ispressurized by the expanding gas. Equal volumes of water and expandedgas pass through a heat exchanger so that there is always sufficientheat energy for the gas to expand. The temperature drop in the water dueto contact with the expanding gas is small. In another aspect, verylarge quantities of CG from a CAeS plant are used to cool a companionnuclear reactor by expansion of the compressed gas alone inside thereactor during emergencies. This aspect provides great cost savings tothe CAeS during normal power generation that pays for the CAeSinstallation near a companion nuclear power station. A CAeS is arelatively inexpensive power plant (3% or less of the cost of a nuclearplant) that can be installed outside existing or new nuclear powerstations.

The NCAP embodiment provides greatly enhanced safety for a nuclear plantand major cost and pollution reduction for a companion CAeS plant thathave heretofore not been available. The cost reduction and improvedprofitability for the CAeS plant pay for combining the two power plantsin one location as described in connection with this embodiment. TheNCAP is a passive safety system in that it does not require any otherexternal power supply or energy source other than the compressed gasstored in the CAeS plant. The NCAP is a passive safety system that doesnot require any other external power supply or energy source.

There is an additional advantage inherent in the NCAP embodiment. If theECCS safety systems in a nuclear plant are operational but the plant haslost its emergency backup power (as happened at Fukushima), thecompanion CAeS system can immediately supply the backup electrical powerneeded by the ECCS systems in the companion nuclear plant.

The examples used herein are calculated to meet the emergency reactorcooling requirements for a 1300 MW electrical nuclear power station witha nuclear reactor that produces 3900 MW of thermal heat energy. Thedecay heat from the reactor fuel rods immediately after the reactorshutdown is assumed to be 7% of the rated 3900 MW thermal, equal to 273MW immediately after shutdown, reducing to 1.5% after one hour. Thesevalues are used for explanatory purposes and are not intended to belimiting in any way. Those skilled in the art of nuclear power plantdesign will understand how to scale the emergency cooling systemdescribed herein to fit smaller and larger nuclear power plants.

First Embodiment FIG. 1

FIGS. 1 to 4 show aspects of one CGES embodiment. In one aspect of thisembodiment, unpressurized water in a large tank is pressurized bycompressed gas that is kept at high pressure in a separate tank. Thewater is pressurized only when it is needed during a nuclear emergency.The heat required for rapid expansion of large volumes of the compressedgas is taken from the pressurized water as it circulates around anexpansion valve and coil combination and through a heat exchanger. Sinceequal volumes of water and expanded gas pass through the heat exchangerin a given time, the temperature drop in the water is small.

FIG. 1 shows a schematic diagram of one aspect of a CGES system, whileFIG. 2 shows one physical realization of the system of FIG. 1. Thesystem comprises a nuclear power plant with a reactor pressure vessel(RPV) 102 that is housed within a primary containment vessel (PCV) 104.A first storage reservoir tank 106 contains water 108 at ambientpressure 110. A second storage tank 112 contains CG at high-pressure114, a heat exchanger 116, and a plurality of valves and interconnectingpipes. Tanks 106 and 112, heat exchanger 116, and a portion ofinterconnecting pipes are located beneath ground level 118.

A pipe 120 extends from tank 112 to a valve 122 and then to a pointabove ground level 118. Another pipe 124 connects to pipe 120 at a pointbetween valve 122 and tank 112. Pipe 124 connects pipe 120 to heatexchanger 116 via another valve 126.

Heat exchanger 116 comprises an interior volume and has a pressureregulator and expansion valve 128 and a coil 130. Valve 128 is one ofmany standardized pressure regulator and expansion valve designscommonly used in the compressed gas industry. Valve 128 is connected topipe 124 at the right-hand side of valve 126. Pipe 138 deliverspressurized water from the bottom of tank 106 to volume 132 in heatexchanger 116. Additional pipes 136, 138, and 140 extend from heatexchanger 116 to tank 106 and plant 100. Pipe 136 extends from coil 130in heat exchanger 116 to the top of tank 106. Pipe 138 extends from apoint near the bottom of tank 106 to volume 132 in heat exchanger 116.Pipe 140 extends from internal volume 132 of heat exchanger 116 to plant100 via a valve 142.

Expansion valve 128 is shown inside heat exchanger 116 to depict that itis constantly surrounded by water in heat exchanger 116. Valve 128 is ofsufficient design and surface area to absorb the heat needed to keep itscomponents at acceptable operating temperatures. Coil 130 depictsadditional surface area for heat transfer that may be an integral partof valve 128.

Pipes 120 and 134 extend above ground for access to CG for tank 112 andwater for tank 106.

After their usefulness in a nuclear reactor is over, nuclear fuel rodsstill generate heat from nuclear decay and are usually stored in acooling water bath to prevent overheating. Spent fuel rods are removedfrom reactor 100 and stored in a spent-fuel pool (SFP) 144. Emergencycooling water is delivered to pool 144 by a pipe 146 when valve 148 isopened.

Pipes 120, 124, 134, 136, 138, and 140 are securely and leaklesslysealed to tanks 106 and 112, heat exchanger 116, and plant 100 at theirpoints of entrance and exit.

Operation—FIG. 1

In FIG. 1, the external ECCS system pumps cooling water into plant 100during a nuclear emergency to prevent a meltdown when in-plant EmergencyCore Cooling systems are disabled or damaged. It can inject largevolumes of cooling fluid immediately into a reactor 102 even if there isstill pressure in the reactor vessel (RPV) 102 so long as the pressurein tank 106 is greater than the water-steam pressure in RPA 102. Thepressure in tank 106 can be as high as 20.7 bar (300 psi) for a tank 106holding over 100 m3 of cooling water (see design example below). One ofthe most dangerous situations in a nuclear plant is sudden loss ofcooling water in the RPV 102 and the nuclear fuel rods therein are nolonger covered by cooling fluid. The RPV pressure can drop well below20.7 bar. The CGES system can immediately inject cooling water at a rateas high as 6500 l/m to absorb all the decay heat from a 1300 MWe (3900MW thermal) nuclear reactor (273 MW initial rate after SCRAM). A fullECCS system to provide up to three days of reactor cooling will comprisemany CGES modules as shown in FIG. 1.

Valve 126 is first opened manually or automatically allowing highpressure gas (CG) from tank 112 to enter expansion and pressureregulator valve 128 in heat exchanger 116. Gas flows from valve 128,through coils 130, through pipe 136, and into tank 106. The pressureexerted on the surface of the water in tank 106 urges the water upwardthrough pipe 138, into volume 132 of heat exchanger 116, and out intopipes 140 and-or 146. When valve 142 is manually or automaticallyopened, water flows outward through pipe 140 and onward to manifold 300of plant 100. The water delivered under pressure to manifold 300 is sentto reactor pressure vessel 102 or primary containment vessel, 104, asneeded, by selectively manually or automatically operating valves 305,310, and 325 (FIG. 3).

FIG. 2—Description

FIG. 2 is a cross-sectional view that shows a physical example of oneaspect of the system of FIG. 1. FIG. 2 shows additional components thatstrengthen the design in FIG. 1 in order to contain the high pressuresduring both storage and operation.

The present aspect shown is an underground version of the CGES system.In this embodiment, CG tank 112 is mounted inside cooling water tank106. This is an embodiment that allows both tanks to be contained in onebore hole that is constructed by standard oil-gas well drillingtechniques.

When valve 126 is open, CG from tank 112 flows into pressure reductionvalve 128. Control device 260 adjusts the pressure of CG leaving valve128 to keep the pressure in tank 106 below the maximum allowable level.

Valve 128 is manually or automatically adjusted to set the pressure ofCG from tank 112 entering coils 130 of heat exchanger 116. Safety reliefvalve 255 assures that the maximum allowable pressure in tank 106 is notexceeded. Adjustable flow rate valve 142 controls the rate ofpressurized water leaving tank 106 which determines the rate of expandedCG allowed into tank 106 when valve 126 is open. Water flowing from tank106 through volume 132 of heat exchanger 116 provides heat needed toprevent freezing of components in 116 or water in tank 106 by theexpanding gas from tank 112.

A tank 106 is formed within a borehole of predetermined depth anddiameter in the earth. The tank has a cylindrical steel casing 200 thatis open at both ends and has a diameter less than that of the borehole.During installation, casing 200 is lowered into the borehole andsecurely held a short distance above the bottom. Standard groutingprocedures from in the oil-gas well drilling industry are used to insertconcrete grouting material 205 between casing 200 and the bottominterior of the borehole to form a concrete plug 210 at the bottom ofthe borehole and grouting 205 around casing 200.

Grouting 205 and plug 210 are of sufficient size and composition toprevent significant leaking of fluids contained therein or damage tocasing 200 at the operating fluid pressures utilized. A lid 215 closestank 106 at the upper end of casing 200. A gasket 220 or other sealantmeans is placed between the top of casing 200 and the bottom surface oflid 215 to prevent gas leakage when the present system is activated.

A number of concrete blocks 225 are placed atop lid 215 to further weighit down against the top of casing 200 and gasket 220, thereby forming atight seal for tank 106. A plurality of bolts 230 further secure lid 215to casing 200. In this aspect, bolts 230 bear against brackets 235 thatare secured to casing 200 by a plurality of welds 240. When bolts 230are tightened, lid 215 is clamped securely in place on tank 106.

At the upper right of FIG. 2, an additional inlet pipe 245 is connectedto pipe 134A via a valve 250. Inlet pipe 245 is connected to an outsidesource such as reactor pressure vessel (RPV) 102, primary containmentvessel (PCV) 104, spent-fuel pool (SFP) 144 or another source for returnor delivery of water to tank 106. Water is also delivered to tank 106through pipe 134A via valve 136A, heat exchanger 116, and pipe 138.

A vent line 290 and a valve 295 are used to vent air from tank 106 whenfluid is added via pipes 134 and 138 or pipe 245. Valve 295 is opened tovent displaced air from tank 106; otherwise it is closed.

The fluid returned from plant 100 to tank 106 can be the hot fluidgenerated in the RPV from the cooling water initially injected by thistank 106 or other tanks 106 into plant 100. In this manner, the reactorcooling capacity of water initially stored in tanks 106 can be extended.

A few days after SCRAM, the decay heat rate falls below 0.4% of ratedreactor thermal power. When the heat transfer rate through the casing200 of one of more tanks 106 equals or exceeds the decay heat rate in areactor, a CGES system comprising several tanks 106 can cool the reactorindefinitely by recycling the cooling water in tanks 106.

Underground tank 106 that is in a deep bore hole as shown in FIG. 2 canmaintain a substantial heat transfer rate through its steel casing 200and concrete grout 205, especially if the bore hole passes throughunderground aquifers where water is in constant contact with the outsidegrouting 205.

The above cooling water recycling capability of the resent CGES systemis important in comparison to the AP1000 system, supra, since it cannotreturn a reactor to cold condition, where the coolant less than 100° C.The AP1000 must have continuous hot fluid in the reactor vessel tomaintain circulation of the coolant through a heat exchanger that usesthe gravity water from the AP1000 system tank to absorb decay heat fromthe hot primary fluid. In contrast, by recycling its cooling water asdescribed, the CGES can cool a reactor below 100° C. and maintain thatcondition long-term.

Moreover the CGES does not require Sato's DPCS apparatus, supra, tobring the reactor to cold shutdown condition. Both the AP1000 and Sato'sapparatus use reactor primary coolant circulation to cool the reactor.They are critically dependent on plumbing internal to the nuclear plant,but such plumbing can be disabled by attacks by nature or terrorists. Incontrast, the present CGES cooling operation does not use the primaryboundary plumbing connected to the reactor pressure vessel (RPV). TheCGES system is effective even if the internal plumbing is damaged andcannot retain coolant in the primary boundary. The CGES system injectsnew coolant into a reactor through the external coolant input port andconstantly replaces the coolant in the reactor anytime it is notcovering the fuel rods in the reactor. The CGES coolant is constantlyevaporated to absorb maximum decay heat and then vented out of thereactor pressure vessel (RPV) as water vapor, which is then replaced bymore CGES coolant. The CGES system works best when any hot primarycoolant is vented out of the RPV after a SCRAM so that the injected CGEScoolant is not wasted absorbing the heat from the left-over primarycoolant.

The safety pressure release valve 255 below lid 215 allows venting ofexcess pressure within tank 106 whenever the pressure exceeds a pre-setmaximum allowed pressure (usually less than 21 bar for a tank 106greater than 1 meter in diameter). A pipe 260 extends from valve 255 tothe air above ground level for venting. In various aspects an embodimentcan have two or more safety relief valves 255 mounted on tank 106 forredundancy in this critical component that protects tank 106 fromexcessive pressures.

In the present aspect, gas expansion valve 128A includes a handle 260for adjustment of the output pressure of valve 128A.

Valves 126, 142, 148, and 270 all are one-way (check) valves that allowflow only in the direction of the arrows shown in the respective lines.These check valves prevent backflow into the lines in case of excessivepressures on the other side of the valve, i.e., as in line 140 whereexcessive pressure in the RPV or PCV could force water back into tank106 and damage it. Pipe 265 allows the CG in tank 112 to be directedelsewhere when valves 126 and 270 are open. For example, when the CG intank 112 is nitrogen, the nitrogen can be used to recharge nitrogensupplies that are used to operate valves in a nuclear plant or to flooda reactor vessel with inert nitrogen to reduce known hazards ofoxidation of metals in the fuel rods and to prevent an explosion ofhydrogen gas.

Exemplary Design Specifications and Dimensions for Tanks 106 and 112.

The following are specifications for tanks 106 and 112 in an exemplaryCGES system.

1. Tank 106 has a volume of 100 m³ (cubic meters).

2. Tank 112 has an initial CG pressure of 200 bar (3000 psi) and amaximum pressure of 20.7 bar (300 psi) in tank 106 during a nuclearemergency to limit internal hydraulic forces in tank 106 (as controlledby valve 128A in FIG. 2).

3. The volume ratio of tank 106 to tank 112 is 10:1 (100 cubic meters ofwater to 10 cubic meters of CG).

4. Cooling water must be delivered from tank 106 at a rate of 6552 l/m(1731 gpm) to handle the maximum decay heat rate of 273 kJ per sec (7%)from a 3900 MW thermal reactor immediately after SCRAM (assuming 2,500MJ per m³ heat absorbed by injected cooling water raised from 20° C. tofull evaporation in the reactor).

5. Tank 106 has an inside diameter of 1.19 m (47 in) and is 100 metersdeep with a gross volume of 112 m³. The wall thickness of tank 106 is0.95 cm (0.375 in). The critical stress in the wall of tank 106 withinternal gas pressure 20.7 bar (300 psi) is 1,296 bar (18,800 psi).(This assumes no support from the concrete grouting outside the wellcasing in the case that tank 106 is in a borehole.)

6. Tank 112 has an inside diameter of 0.357 m (14.06 in) and is 100meters deep to yield a volume of 10 m³. The wall thickness of steel pipefor tank 112 must be at least 2.68 cm (1.054 in) to keep the criticalstress in the wall of this tank no greater than 1,379 bar (20,000 psi)for internal gas pressure of 20.7 bar (3000 psi).

7. The maximum expansion of the CG from tank 112 will be 11:1 when tank106 is empty. The final pressure in tank 106 will be 200 bar divided by11=18.2 bar (264 psi). This is enough remaining pressure to lift thelast water in tank 106 up to a height of 186 m (609 ft).

8. At a pressure of 20.7 bar (300 psi) in tank 106 with a diameter of1.19 m (47 in), the upward force on lid 215 (FIG. 2) is 2,315,300 N(520,500 lb). The 1.19 m diameter casing 200 of tank 106 with 0.95 cmthick steel wall can resist an upward force of 4,525,542 N (1,017,400lb) with a maximum 1,379 bar (20,000 psi) stress in the casing wall whenlid 215 is securely attached to casing 200. This is 2.1 times the actualload force in the casing wall due to the upward force of 2,315,300 N,for a safety factor of 2.1. The pressure in tank 106 could increase to2.1 times 20.7 bar=43.5 bar and not exceed the stress limit of 1,379 barin the wall of tank 106.

9. Concrete weight 225 on tank 106 should be 444,800 N (100,000 lb) asan added safety factor to resist the upward hydraulic force on the topof tank 106 and reduce the load on the outer part of lid 214. Fifty tonsof concrete requires 19.12 m3 (25 cu yd) of concrete at 11,518 kg per m3(4,000 lb/cu yd).

A heat transfer rate of 10.25 KW into the CG is required for it toexpand by 10:1 at a rate of 0.1092 m3/s (1800 gpm) to propel water outof tank 106 at the same volume rate. 25 kW is 25 kJ/s taken from thewater leaving tank 106 at the rate of 0.1092 m3/s. The specific heat ofwater is 4.18 MJ/m3/deg C. The CG takes only 25 kJ from each 0.1092 m3of water that flows through heat exchanger 116. This is a rate of 228kJ/m3 of water. The temperature drop per m³ of water is 228 kJ/m3divided by 4.18 MJ/m³/° C.=0.05° C. Thus the water temperature drops byfar less than 1° C. as it flows out of tank 106.

A person skilled in the art of physics or engineering can scale theabove design example to other workable dimensions for tanks 106 and 112and CG pressures as appropriate. All metal components shown in FIG. 2are steel or another alloy that is sufficiently strong to meet thedesign requirements of the CGES and are not susceptible to corrosion inthe environment shown.

Connection of CGES to Plant 100—FIGS. 2 and 3.

FIG. 3 shows a schematic diagram of a nuclear power plant thatillustrates paths that cooling fluids take after SCRAM. When a CGES isactivated, water leaves tank 106 (FIG. 2) and is delivered to plant 100via pipe 140 or inlet manifold of plant 100. A shut-off valve 305 onmanifold 140 is opened to admit water into plant 100. A pipe 315conducts water to a valve 310. A pipe 320 branches from pipe 315 andconducts water to a valve 325. When valve 310 is open, water is directedinto reactor pressure vessel 102. When valve 325 is open water isdirected into primary containment vessel 104. Either or both of valves310 and 325 can be opened, depending on cooling requirements after aSCRAM in plant 100.

Additional valves 330 and 335 are opened as appropriate to conduct waterand steam from reactor pressure vessel 102 and primary containmentvessel 104 to a manifold 350 within plant 100. Manifold 350 is connectedto pipe 245 (FIG. 2) so that water and steam from plant 100 can bereturned to tank 106 if desired.

First Embodiment Operation—FIGS. 1 to 3.

In the following example a CGES module as shown in FIG. 2 has beeninstalled and connected to a plant 100 (FIG. 3). All abovegroundconnections to the CGES are sealed to prevent unwanted matter fromentering the system. All valves in the CGES system are initially closed.

Preparing CGES for Use.

A source of water (not shown) is connected to pipe 134A and valve 136Ais opened. A predetermined amount of water flows through pipe 134A andvolume 132 of heat exchanger 116 and into tank 106. When the amount ofwater supplied fills approximately 95% of tank 106, valve 136A isclosed, stopping the delivery of water to tank 106.

A source of high-pressure gas (air, nitrogen, or other gas) is connectedto pipe 120 and valve 122 is opened. Gas is urged from the source intotank 112. When a predetermined pressure is reached, valve 122 is closed,stopping the delivery of gas into tank 112.

When the delivery of gas and water to the CGES system is complete, thesources of gas and water are optionally removed or left in place.

Activating CGES after SCRAM.

As with the system of FIG. 1, assume that a disaster occurs so that areactor 102 must be shut down under emergency conditions (SCRAM). Toprevent a meltdown, the present system pumps cooling water into plant100. Valve 126 is first opened manually or automatically allowing gasfrom tank 112 to enter expansion and pressure regulator valve 128A inheat exchanger 116. Gas flows from valve 128A, through coils 130,through pipe 136, and into tank 106. The pressure exerted on the surfaceof the water in tank 106 urges the water upward through pipe 138, intovolume 132 of heat exchanger 116, and out into pipe 134A. When valve 142is opened manually or automatically, cooling water flows outward throughpipe 140 and onward to manifold 300 of plant 100. The water deliveredunder pressure to manifold 300 is sent to reactor pressure vessel 102 orprimary containment vessel, 104, as needed, by selectively operatingvalves 305, 310, and 325 (FIG. 3) manually or automatically.

Valve 128A control device 260 is manually or automatically adjusted tocontrol the pressure of CG leaving valve 128A so that it does not exceedthe maximum pressure allowed in tank 106.

Other CGES Activities—Delivering Water to a SFP.

A pipe 146 (FIG. 3) is connected to a SFP (not shown) for delivery ofcooling water to any spent fuel rods that still give off heat and mustbe cooled when normal plant 100 cooling water circulation is disabled.Starting with all valves closed and sufficient gas in tank 112 and waterin tank 106, valves 126 and 260 are opened and valve 260 is adjusted sothat pressure inside tank 106 is sufficient to urge water to flow outpipe 146. Valve 148 is then opened and water flows to SFP from tank 106until either tank 106 empties or sufficient water is delivered. Valve148 is then closed and gas and water supplies to the CGES are restored,if necessary.

Recovery and Recycling of Water and Steam from Plant 100.

In FIG. 2, a pipe 245 is connected to outflow manifold 350 of plant 100.Initially, all valves are closed. When it is desired to return waterfrom plant 100 to a tank 106, valve 250 is opened and water and-or steamflows from plant 100 back into tank 106. This feature allows the ventingof the initial high pressure water and steam in a SCRAMMED nuclearreactor 102 back into empty tanks 106 instead of venting the radioactivesteam into the environment. This is extremely important when it isnecessary to inject cooling water into a reactor 102 that has reachedits maximum heat content and internal pressures because it cannotcirculate and cool its existing cooling fluid. By this means, thepressure and water-steam in a reactor 102 can be reduced to allowinjection of new cooling water from tanks 106.

Return pipe 245 also allows the CGES embodiment in FIGS. 2-5 to performlong-term cooling of a reactor 102 beyond the critical first days afterSCRAM. Cooling water from a tank 106 that is heated in and thenevaporated out from a reactor 102 can be returned to other empty tanks106 for later reuse as cooling water. In this manner, the coolingcapability of an initial amount of water in a set of tanks 106 can beextended indefinitely after the decay heat rate in a reactor 102 dropslow enough so that cooling water from tanks 106 can be recycled andcondensed in empty tanks 106 faster than it is needed to cool reactor102. In particular, deep underground tanks 106 as shown in FIG. 2 canprovide substantial cooling for water and-or steam recycled from areactor 102 as described above. This CGES embodiment eventually can coola reactor below the desired “cold shutdown” condition whereby thetemperature within the reactor is less than 100 degrees C.

The CGES system is a substantial improvement over the AP1000 system. TheAP1000 system requires continuous primary cooling water circulationthrough a reactor. The CGES embodiment can cool a reactor even after itsprimary cooling water has been lost.

Delivery of Gas to Another Location.

A pipe 265 in FIG. 2 branches from pipe 124 and is arranged to delivercompressed gas to other locations. For instance, if the compressed gasis nitrogen, the nitrogen can be delivered to storage tanks within anuclear plant 100 for nitrogen actuated control valves. When it isdesired to deliver gas through pipe 265, valves 126 and 270 are openedand gas from tank 112 exits the CGES via pipe 265. Closing either valve126 or 270 stops the delivery of gas.

First Alternative Embodiment Description and Operation—FIG. 4

FIG. 4 is a schematic plan view of one aspect in which a plurality ofCGES units 400 are connected to a nuclear plant 100. This arrangementprovides redundancy in case one or more CAES units are damaged orinaccessible.

A plurality of pipes 405 connect the water storage volume in each oftanks 106 (FIG. 1) to that of its neighbors. Another plurality of pipes140 are arranged to deliver water to plant 100 when required after aSCRAM, as explained above. An additional plurality of pipes 245 arearranged to pass water and steam back from plant 100 to CGES units 400.Manual on-off valves 410 are installed at the beginning and end of eachpipeline segment so that a damaged or leaking pipeline segment can beisolated by closing the appropriate valves 410. Some or all of valves410 are automatic if required. Additional pipes 120 and 134A permit CGand water from external sources to be injected into tanks 112 and 106,respectively.

Additional tanks 415L and 415R contain liquid nitrogen that is deliveredto pipes 120 via pipes 420L and 420R, respectively. The liquid nitrogenin these tanks is used to recharge tanks 112 106 (FIG. 1) as necessary.Additional tanks 420L and 420R are above-ground, standard water storagetanks of the type commonly employed by water utility districts for bulkstorage of water. Their capacity is typically 3,700 m³ (1,000,000 gal)or more. Tanks 420L and 420R replenish the water in tanks 106 in CGESunits 400 (FIGS. 1 and 2) for long-term reactor cooling operations afterthe critical first days after reactor shutdown.

Valve 425L controls water flow from tank 420L to a tank 106 in a CGESunit 400 by line 134A. Valve 605R controls water flow from tank 420R tothe input line 134A to a tank 106 in a CGES unit 400. Many tanks 420L or420R can be stationed around a nuclear plant and connected as shown toany or all of tanks 106 in CGES units 400. Water is stored in tanks 420at ambient atmospheric pressure, and flows from tanks 420 to tanks 106by gravity. When no pressure is in reactor 102 and the reactor isunderground below the level of tanks 420, water from these tanks can besent directly to the reactor 102 by appropriate settings of pipelinevalves shown in FIG. 4.

All pipes are placed on or under open buffer areas or beneath parkingareas around plant 100 so that there is no loss of surface space aroundplant 100.

Second Alternative Embodiment Description and Operation—FIGS. 5 to 7

Above Ground Versions of the CGES System—First Version.

FIGS. 5 to 7 show aspects of an embodiment of the CGES system that usesstandard railroad (RR) tank cars for transport an above-ground versionof water storage tank 106 (FIG. 2). With CG tanks 112 attached to eachtank car or supplied separately, this provides a simple, inexpensive,and very reliable CGES system that can be installed immediately at mostany nuclear power station. This CGES system embodiment is called a RRTC(Railroad Tank Car) system. The RR tank car CGES modules compriseportable units that can be transported from one location to most anyother nuclear plant in trouble within 24 hours.

FIG. 5 is a schematic view showing a plurality of railroad tank cars500A through 500D in the vicinity of a nuclear plant 100. Cars 500 aremoved on a plurality of track segments 505 that are located atpredetermined positions and distances in the vicinity of plant 100.Although four tank cars are shown, fewer or more can be used.

Railroad tank cars can hold up to 128.7 m3 (34,000 gal) of liquid, withsome older tank cars holding up to 189.27 m3 (50,000 gal). Although railcars 500 serve the function of tank 106 (FIG. 2), they have greatercapacity than the 100 m³ capacity of tank 106 used in the designcalculation example above. A plurality of tanks 510 are mounted on eachrail car. Tanks 510 serve the function of tank 112 (FIG. 2). In oneaspect, each of tanks 510 is a 15.2 m long, 0.36 m in diameterhigh-pressure steel pipe, and there are between 6 and 8 tanks 510 oneach rail car. The combined volume of the 6 to 8 tanks 510 is aboutequal to that of tank 112 used in the design example above to provide atleast 10 m³ of CG at 200 bars (2900 psi) of pressure.

Railroad tank cars can withstand internal gas pressures up to 6.9 bars(100 psi). That is more than enough pressure to force water at high flowrates into plant 100 during a nuclear emergency.

In FIGS. 5 to 7, the same pipelines and valves that interconnect CG tank112 to water tank 106 in FIG. 2 (with the same reference numbers) areused to connect tanks 500 and 510. Although present as indicated in FIG.2, valves, other than valve 126, are not shown in FIGS. 5 to 7.

In FIG. 5, tank car 500A is expanded to better show the components usedin this aspect of the present embodiment. The rail car tank on cars 500is equivalent to tank 106 in the first embodiment. Tanks 510 areconnected to a sealed manifold 515 by a plurality of pipes 520. Tanks510, pipes 520, and manifold 515 are collectively equivalent to tank 112in the first embodiment. When valve 126 is opened, CG flows frommanifold 515, through pipe 120 and valve 126 into expansion and pressureregulator valve 128A (FIG. 2), indicated by pressure adjusting handle260, through heat exchanger 116, and out via pipe 136 to pressurizedtank 500A. Pressurized water flows out line 138 from tank 500A into heatexchanger 116 and then out pipe 140 to plant 100. Thus pressure isapplied to the interior of tank 500A via pipe 136 and water is urged toleave tank 500A via pipe 138, passing through volume 132 of heatexchanger 116, and finally on to plant 100 via pipe 140.

The RRTC system sacrifices some protection against assault by acts ofnature or terrorism that is inherent in the underground CGES system ofthe first embodiment. However the immediate availability of manyrailroad tank cars and flat cars around the industrialized world makesthe RRTC system a very practical and inexpensive CGES that can beprotecting a nuclear power plant within a few months from the start ofinstallation, at very little cost. The RRTC has all the operationalcharacteristics of the other CGES embodiments described above.

Second Version—FIG. 6

FIG. 6 is a schematic diagram showing an alternative form of the tankcar system of FIG. 5. In this aspect a larger CG storage tank comprisinga plurality of tanks 510 serves a plurality of rail cars 500E and 500F.Although two cars are shown, more can be added. Tanks 510A are a stackof pipe sections that terminate in sealed manifolds 515A and 515B ateach end of the stack. Tanks 510A are mounted on a flat rail car 600. CGis released from tanks via pipe sections 124 when valves 126 and 128(not shown in this drawing) are opened. When the CG is released, CGpressure is applied to tanks 500 via pipes 136, water is forced out oftanks 500 through pipes 138, through heat exchangers 116, and outthrough pipes 140 to nuclear plant 100 (not shown in this figure).

To provide enough CG to pressurize at least six water storage tanks 500,40 to 60 pipe sections 510A with dimensions described above should beprovided. In the present arrangement, many railroad tanks 500 can bebrought near flatcar 600 with tanks 510A and connected to lines 136,138, and 140. When a water tank car is empty, it can be quicklydisconnected from the CG source on flatcar 600 and replaced by another,thereby providing a continuing source of water to plant 100.

Third Version—FIG. 7

FIG. 7 shows a third version, specifically a cross-sectional end view ofa housing 700 that protects and conceals an RRTC. A safe parking andconnection area for the RRTC tank cars and flat cars placed around anuclear plant can be as simple as a shallow trench that has concretewalls on either side to protect the railroad cars from the high winds ofa tornado and terrorist attacks with vehicles. Empty cars can be rolledaway during a nuclear emergency and either refilled or replaced bypreviously filled cars.

Tank cars 500 and flat cars 600 with CG storage tanks 510 are parked ina trench 705 dug into the ground to partially protect the RRTC system. Astrong concrete fence 710 runs along both sides of trench 705. A roofstructure 715 sits on fence 710 and spans across trench 705 above therail cars. A plurality of railroad tracks 720 are laid on the ground atthe bottom of trench 705.

This embodiment of the RRTC system provides substantial protection fromtornadoes and vehicular attacks by terrorists. The below ground levelplacement of tracks 720 also allows easy movement of replacement tankcars 500 and CG storage cars 600.

This RRTC system is not expensive to construct. It requires mainly dirtexcavation and placement of prefabricated fence sections 710 along theedges of trench 705. Fence sections 710 can be standard K-rail concretesections used as traffic barriers on roadways, for example.

Full CGES System for Large Nuclear Plant

A full CGES Emergency Core Cooling System using the CGES embodimentsshown in FIGS. 2-4 and FIGS. 5-7 will normally comprise manyinterconnected water tanks 106 and CG tanks 112 as shown in FIG. 4. Forinstance, the total decay heat from a 1300 MWe (3900 MWt) reactor forthe first three days after SCRAM is about 5.5 GJ (5500 MJ). Absorbingthis decay heat requires at least 22 tanks 106 each holding 100 m3 ofcooling water plus as many companion tanks 112 holding 10 m3 of CG at200 bar (or equivalent CG volume stored by other means). Each 100 m3 ofcooling water in a tank 106 can absorb 0.25 GJ of decay heat when thewater is heated to full evaporation from 20 degrees C. Twenty two timesthis will absorb the full 5.5 GJ) of decay heat for the first threedays, assuming no recycling of the cooling water in the tanks 106. Asmaller nuclear plant, say, 1000 MWe, would require only 17 tanks 106 of100 m3 capacity each.

The tanks 112 sources of CG shown in FIGS. 1 and 2 can be realized inmany forms other than those shown in the embodiments described herein.Tanks 112 can be separate bore holes lined with steel nearby the tanks106 that they pressurize. Above ground tanks 112 can be segments of highpressure pipe, such as used for high pressure natural gas pipelines. Oneof the least expensive ways to store and deliver large quantities of CGis to use many segments of small diameter, thick-walled pipe stackedtogether and connected as shown for item 510A on flatcar 600 in FIG. 6.CG tanks can be realized with large capacity plastic bags or bladdersthat are filled with CG and held deep underwater to balance the internalpressure of the CG. An advantage of this approach is that the CG isdelivered at constant pressure equal to the water pressure on the bags.

Inherent Safety of Compressed Gas Supply Systems.

The CG energy sources for all embodiments of the CGES system can bereplenished by external or emergency air compressors and compressed airsources such as the liquid nitrogen tanks 415L and 415R shown in FIG. 4.Attaching additional sources of compressed gas to an existing CGdelivery system is both far easier and safer than attaching backupelectrical generators to an existing power grid. Backup generatorsattached to a nuclear plant must be matched carefully in voltage andphase with the internal electrical system. However, using check-valveconnectors, additional sources of compressed air, such as tanks 112 inthe above embodiments, can be attached easily to an existing CG supplysystem with reduced concerns for safety or compatibility. CG will flowfrom the new system to the existing supply system only when the newsystem pressure is greater than the pressure in the existing system. Theimportance of this feature of the CGES system was demonstrated by thefrustration of operators during the Fukushima accident in 2011 when theytried to connect outside backup electrical generators to the nuclearplant electrical switching equipment that had been disabled by thetsunami.

Third Alternative Embodiment Description and Operation—FIG. 8

FIG. 8 is a schematic diagram showing a combination of a nuclear powerplant 100 with a Compressed Air Energy Storage (CAeS) power plant 800.In this embodiment as shown in FIG. 8, a nuclear plant 100 and a CAeSplant 800 are connected in a unique way that provides a passive safetysystem for the nuclear plant 100 and a great cost savings and pollutionreduction for the CAeS plant. System 800 stores a very large amount ofcompressed air (CG) in an underground storage cavern 805 which isnormally used to generate power during peak power demands on anelectrical grid to which it is connected. Cavern 805 is recharged withCG during off-peak times. The large amount of CG in cavern 805 is alsoused to cool the reactor in its companion nuclear plant 100 during anuclear emergency. This embodiment is called NCAP for Nuclear CompressedAir Power plant.

In one aspect of the NCAP embodiment, the CAeS system comprises a heatexchanger 810 having an interior volume containing a coil 812, a turbine815, an auxiliary generator 820, a compressor 825, cavern 805, and aplurality of pipes and valves. A pipe 830 conveys CG to and from cavern805.

Storing CG in CAeS Plant.

Compressor 825 is connected to pipe 830 by a valve 835. When it isdesired to add CG to cavern 805, valve 835 is open, compressor 825 isactivated, and all other valves are closed. Compressor 825 can bepowered by the electrical output of plant 100, or another means such asan internal combustion engine. When the CG pressure in cavern 805reaches a predetermined level, compressor 825 is deactivated and valve835 is closed.

Cooling Plant 100 in an Emergency—FIG. 8.

In the event of a SCRAM, RPV 102 can be cooled very rapidly by the CGstored in the companion CAeS plant 800 shown in FIG. 8. A flow controlvalve 870 and pipe 875 are connected to CG transfer pipe 830 for thispurpose. In this event, flow control valve 870 is opened manually orautomatically to send CG directly from storage in cavern 805 to expandaround the fuel rods in the nuclear reactor 102 in plant 100. Flowcontrol valve 870 is mounted on or near the reactor pressure vessel 102so that it can extract heat from the reactor vessel as required forproper operation.

Use of CAeS Plant to Supply Emergency Power for Companion Nuclear Plant.

In normal operation, nuclear power plant 100 generates steam that issent via a pipe 802 to a turbine 840 that in turn drives an electricalgenerator 845. Generator 845 delivers electrical power to a power gridvia a plurality of conductors 850. Plant 100 receives electrical powerfrom the power grid and-or backup electrical generators via a pluralityof conductors 855.

During an emergency when backup electrical power is disabled but theemergency reactor cooling systems (ECCS) in plant 100 are otherwisecapable of operating, the electrical output of CAeS plant 800 can veryquickly supply emergency power to plant 100.

Generator 820 has a plurality of output conductors 822. When required,lines 855 that feed plant 100 are disconnected from an external powersource and connected to lines 822 of generator 820. To power turbine 815and generator 820, a valve 832 in pipe 830 is opened so that CG entersheat exchanger 810, passes over a set of coils 812 in heat exchanger810, and exits via a pipe 814 on its way to turbine 815.

A typical nuclear plant 100 that is disconnected from the electricalgrid cannot supply power thereafter to its own emergency systems. Thepower output of the nuclear reactor cannot be scaled down quickly to themuch smaller demand of its own emergency systems alone. However, a CAeSpower plant 800 is a rapid response peak power plant that can scale itsoutput power to the power requirements at any time. Hence, the CAeSpower plant 800 can supply the backup electrical power needed to operatethe ECCS systems in nuclear plant 100 when it is disconnected from thegrid.

Eliminating Fossil Fuel Energy Normally Required by a CAeS Plant.

In the normal operation of existing CAeS power plants 800, large amountsof natural gas or other fuel are used to supply the heat required forthe CG in storage 805 to expand in turbine 815. However, when a CAeS iscombined with a nuclear plant 100 in the NCAP embodiment, abundant wasteheat is available from plant 100 to heat the CG so it can expand inturbine 815. Steam and water leaving turbine 840 in nuclear plant 100must be cooled and condensed before the water is returned to the reactorpressure vessel. The “waste heat” that must be dissipated isapproximately 60% of the thermal power generated by the reactor in plant100. Only a small part of the waste heat from plant 100 is sufficient toheat the CG from cavern 805 so that it can expand in turbine 815.

In the NCAP embodiment, some of the steam and water leaving turbine 840in nuclear plant 100 are passed through coils 812 of heat exchanger 810,and then returned to plant 100 via a pipe 865. CG passing through heatexchanger 810 collects the heat it needs to expand in turbine 815. Ifmore heat is needed for the CG entering turbine 815, a valve 880,connected to line 802, is opened, sending high temperature steam fromRPV 102 (FIG. 1) directly through pipe 881 and coils 812 of heatexchanger 810.

Gas Path Inside a Reactor—FIG. 9.

FIG. 9 is a schematic diagram that shows one embodiment of the NCAPdesign that injects CG from storage cavern 805 (FIG. 8) in the aboveCAeS system into a nuclear reactor in plant 100. A plurality of fuelrods 900 are positioned within a baffle 905. Baffle 905 is a tall,cylindrical structure that surrounds rods 900. Such baffles are commonlyused to shield the inner wall of RPV 102 from neutron radiation comingfrom the fuel rods and for channeling the primary heat transfer wateraround rods 900 during normal operation. A closed, cylindrical manifold910 is tightly mounted outside and around a lower portion of baffle 905.Pipes 140 and 315 deliver CG to manifold 910 when valve 310 is openedduring a nuclear emergency. During the emergency fuel rods 900 must becooled by external means. CG leaves manifold 910 via a plurality ofopenings 915 and expands in the volume surrounding rods 900. Manifold910 is described in greater detail below. PCV 104 can contain a pool ofsuppression water 920 surrounding RPV 102 up to a predetermined level.

During an emergency when fuel rods 900 must be cooled by external means,valve 310 is opened. A large volume of CG from storage 805 is directedthrough line 315 into baffle 910 inside RPV 102. The CG expands as itflows out of manifold 910 and over rods 900, thereby extracting heatfrom the environment around the fuel rods. The expanded CG exits viapipe 925 either into the atmosphere outside PCV 104 or into PCV 104, orboth. Gases that enter PCV 104 are vented to the atmosphere via pipe 945when valve 950 is opened.

Valve 108 is the safety relief valve on RPV 102 that opens when pressurein RPV 102 exceeds the maximum allowable level. Relief valve 108 can beopened to vent water and steam out of RPV 102 if necessary. When theECCS systems in a SCRAMMED plant 100 are disabled, valve 108 normallywill be opened to vent fluids in RPV 102 and allow injection of coolingfluids into RPV 102. However, this was not done during the Fukushimaaccident until it was too late

A pipe 925 conducts fluids from RPV 102. Valve 930 releases fluids intothe atmosphere when it is opened. Another pipe 935 branches from pipe925. A valve 940 in pipe 935 vents fluids into PCV 104 when it isopened. When desired, valves 925 and 940 can be opened at the same time,venting fluids into PCV 104 and the atmosphere outside RPV 102. Anadditional pipe 945 and valve 950 are provided to vent fluids from PCV104 when valve 950 is opened.

Detailed Description of Baffle 905 and Manifold 910—FIGS. 10 and 11.

FIG. 10 is a top view of manifold 910 and baffle 905. CG enters manifold910 via pipe 315 and flows inward toward rods 900 (not shown in thisview) via a plurality of openings 915.

FIG. 11 is a perspective view of baffle 905 and manifold 910. Manifold910 includes a pair of annular plates 1100 and 1105 that seal manifold910 to baffle 905 in such a way that unexpanded CG is directed to exitout of manifold 910 through a plurality of openings 915 which direct theCG inside of baffle 905.

As above, pipe 315 delivers CG to manifold 910 and manifold 910 directsthe CG to the volume around the fuel rods 900 (not shown in this view).

The feasibility of the NCAP embodiment is demonstrated by the fact thatthe stored CG in a moderate size CAeS plant can cool a 3900 MW thermalreactor for at least three days with no regeneration of the CG in itsstorage 805. One existing 110 MW CAeS peak power plant (not connected toa nuclear plant) stores up to 19 million cubic feet of compressed air at70 bars (1029 psi) in an underground salt cavern (3). This CAeS plantreleases its stored CG to generate 110 MW of electricity on demand forseveral hours per day when needed. The total heat absorbing capacity ofthe 19 million cubic feet of CG at 70 bars is 16 million MegaJoules(MJ), assuming this amount of CG expands isothermally back to 1 bar atambient temperature. (19 million cubic feet=0.538 million cubic meters.Multiplying 0.538 million m³×30 MJ per m³ for air at 1 bar compressed to70 bars=16 million MJ for the heat of compression which heat isextracted from the environment when the gas expands isothermally.)Assuming that only one half of this heat absorbing capacity can berealized by expansion in a reactor, 8 million MJ of reactor decay heatstill can be removed by the stored CG in a typical CAeS plant.

The total decay heat from a 3900 MW nuclear reactor is about 3 millionMJ for the first three days after SCRAM. Even with incomplete expansionof the CG in a reactor, eight million MJ of heat absorbing capacity isenough to cool a 3900 MW reactor for three days with no cooling wateravailable. In addition, the CAeS can begin delivering enormousquantities of CG very quickly, within minutes, to handle the immediatemaximum decay heat (7% of rated thermal power) that must be removed toavoid damage to the fuel rods in a companion nuclear plant as shown inthe NCAP embodiment of FIG. 8.

It can be expected that either grid power or standby electricalgenerators will be available within three days to power air compressor825 in FIG. 8 to regenerate the CG in cavern 805 so that reactor coolingcan continue indefinitely. Note that only the large CG cavern 805 isnecessary to provide a security blanket of emergency cooling for plant100. Plant 800 does not have to produce electrical power from the CG incavern 1080.

The unique nature of the NCAP embodiment is demonstrated by the factthat the nuclear power industry heretofore has not utilized thetremendously valuable safety enhancement plus the great cost savingsavailable by connecting a relatively inexpensive CAeS system to anuclear plant as shown in FIG. 8.

The Fukushima nuclear meltdowns in 2011 would not have happened if thereactors had been connected to even one inexpensive CAeS power plant asdescribed for the NCAP embodiment. Tens of billions of dollars of losscould have been prevented with the addition of the 100 million dollarCAeS plant shown in FIG. 8.

The NCAP combination provides cost savings that pay for the CAeS peakpower plant connected to a nuclear plant. A nuclear plant is the idealoff-peak power source to re-charge the compressed gas storage in a CAeSplant. All the components required for this NCAP embodiment areoperating at separate locations today. Clearly, the two plants should bebuilt at a common location wherever possible. All that is required toachieve the NCAP design is the water-to-gas heat exchanger 810 and apipeline connection 875 between the CAeS plant and nuclear plant 100 asshown in FIG. 8.

The “safety blanket” provided by the NCAP embodiment of FIG. 8 isextremely valuable. This degree of safety has not been available forland-based nuclear reactors by any other means. The cost of a CAeScompressed air underground storage system is less than 3% of the cost ofa typical nuclear power plant. The availability of a (100 million dollaror less) CAeS during a nuclear emergency can avoid the loss of a10-billion-dollar nuclear plant and the tens of billions of additionalcost for environmental damage and clean up, not to mention the loss oflives and injuries and public confidence in nuclear power. And, a CAeSplant connected to a nuclear plant as shown in the NCAP design of FIG. 8pays for itself many times over.

Cooling Fuel Rods in a Ruptured RPV.

The combined nuclear plant and compressed air energy storage (NCAP)embodiment is the only ECCS that can cool a reactor after the RPV hasbeen damaged to the point that it cannot hold any cooling water aroundthe fuel rods therein. In fact, the NCAP works best under this severecondition. A ruptured RPV allows full expansion of the CG injectedaround the fuel rods (and maximum decay heat extraction) without anypressure build up in the RPV. All ECCS systems that rely on coolingwater, including the AP1000, cannot rescue a nuclear power plant with aseverely ruptured RPV.

Serious corrosion has been discovered in the thick steel lid of at leastone older U.S. nuclear reactor RPV. That lid was very close torupturing. Had that occurred under operating pressure, all internal ECCSsystems (and operators) in that nuclear plant would have been disabled.Even a plant using the AP1000 design would have been helpless to avoid ameltdown. With hundreds of aging reactors around the world, it can beexpected that more such events will happen in time.

Compressed Air energy Storage (CAeS) systems are now being planned thatwill use long segments of high pressure pipe on the ground to replaceunderground caverns for storing large quantities of CG. This CG storagecan be installed easily in the open areas around most nuclear powerstations. This embodiment (not shown) of CG storage 805 in FIG. 8provides the NCAR emergency cooling for a reactor even if the CG storedis not used for peak power production as in a full CAeS peak powerplant.

CONCLUSIONS, RAMIFICATIONS, AND SCOPE

It is imperative that the aging nuclear power stations of the world beprovided enhanced safety systems as soon as possible.

All embodiments of the present emergency core cooling systems (ECCS) cancarry out the reactor cooling operation under worst case conditions thatcannot be handled by any of the existing ECCS systems approved or innuclear plants today. These systems can cool a water-cooled reactor inthe most extreme case, i.e., where all primary coolant has been lost andthe internal plumbing connected to the reactor has been damaged suchthat even new cooling water cannot be circulated through the reactor's“primary boundary” by the in-plant equipment. Also reactor pressurevessel (RPV) can be cracked such that it cannot hold high-pressure wateror steam. All embodiments of the present system inject external coolingfluid directly into a RPV under relatively low-pressure conditions. Thecooling fluid injected does not have to be retained long-term in the RPVunder high pressure. The injected coolant absorbs decay heat as itpasses through the RPV. Then it is vented directly out of the reactor tothe primary containment vessel (PCV), and-or empty external water tanks,and-or the environment.

All embodiments can bring a nuclear reactor to cold shutdown conditionand cool it indefinitely thereafter with recharging of cooling fluids.

The systems can be installed immediately from robust, reliablecomponents that are available worldwide without the development andcertification of new technology.

All embodiments are compatible with and can add to the capability ofexisting ECCS systems if the existing systems are capable of operatingto some degree after a nuclear power station has been shut down.

None of the embodiments described require extensive regulatory agencyapproval or delay. They can be installed outside a nuclear power stationwith physical connection only to the existing external cooling waterinput port(s) and output fluid venting port(s) on all nuclear reactorpressure vessels (RPV). They are the equivalent of parking fire trucksloaded with water outside a nuclear plant. Installation can be donewithout significant interruption of nuclear power station operation. Nonew or unproven technology is required to construct any of theseembodiments.

Various embodiments can be installed outside the buildings andinfrastructure of a nuclear plant where they are relatively safe fromdestruction by acts of nature and-or terrorists that can disable theinternal emergency core cooling systems (ECCS) in use today (as happenedat Fukushima in 2011). The embodiments described all contain their ownpower system in the form of stored high pressure compressed gas (CG)that is more reliable and robust than fragile electrical generators(which have failed recently at U.S. nuclear plants) or DC battery banks.The embodiments described are all passive safety systems to at least thesame degree as the AP1000 system approved by the Nuclear RegulatoryCommission (NRC). In contrast, all embodiments shown herein can bring anuclear reactor to cold shutdown condition and cool it indefinitelythereafter with recharging of cooling fluids.

All embodiments of the add-on ECCS can be tested at any time withoutinterruption of power generation at a nuclear plant. A large number ofnuclear plants have been shut down since the Fukushima accident in 2011.Owners are now planning to start up some of these plants. Installationof one or more of the embodiments of the present system can be testedthoroughly on any idle plant before it is reactivated by injectingcooling fluid into the RPV. This would demonstrate enhanced safety tothe press and public in a manner that they can understand.

While the above description contains many specificities, these shouldnot be construed as limitations on the scope, but as exemplifications ofsome present embodiments. Many other ramifications and variations arepossible within the teachings. Thus the scope should be determined bythe appended claims and their legal equivalents, and not by the examplesgiven.

1. A passive safety system for cooling a nuclear power plant aftershutdown comprising: a reservoir of water, said reservoir comprising asealed water tank with at least a first plumbing connection foradmitting compressed gas, and a second plumbing connection fordischarging water from said tank, and a third plumbing connection forreceiving fluids into said tank, a source of compressed gas that isconnected to said tank by said first plumbing connection, at least onevalve in said first pipe between said source of compressed gas and saidtank, said second plumbing connection connecting said tank to saidnuclear power plant, at least one valve in said second pipe between saidreservoir and said nuclear power plant, said first and said secondvalves being normally closed, but openable in case of a shutdown,whereby after shutdown of said nuclear power plant said first and saidsecond valves can be opened so that said compressed gas flows throughsaid first plumbing connection and urges said water to leave said tankvia said second plumbing connection and enter said nuclear power plant,thereby cooling said nuclear power plant, and whereby said third valvecan be opened to allow fluids from said nuclear plant to flow into saidtank when said tank can receive fluids when said first and said secondvalves are closed.
 2. The passive safety system of claim 1 wherein saidtank and said source of compressed gas are buried underground.
 3. Thepassive safety system of claim 1 wherein said tank and said source ofcompressed gas are portable.
 4. The passive safety system of claim 3wherein said tank and said source of compressed gas are mounted on atleast one movable wheeled conveyance.
 5. The passive safety system ofclaim 4 wherein said wheeled conveyance, said tank, and said source ofcompressed gas are concealed in a trench.
 6. The passive safety systemof claim 4 wherein said wheeled conveyance is at least one railroad car.7. The passive safety system of claim 1 wherein at least one of saidtank and said source of compressed gas is located above ground.
 8. Thepassive safety system of claim 1 wherein said system further includes aplurality of said water tanks.
 9. The passive safety system of claim 1wherein said system further includes a plurality of said sources ofcompressed gas.
 10. The passive safety system of claim 1 wherein saidsource of compressed gas is a sealed compressed gas tank incommunication with said water tank via said first valve so that saidsource of compressed gas flows into said water tank only when said firstvalve is opened.
 11. The passive safety system of claim 10 wherein saidsealed compressed gas tank is contained within said water tank andsurrounded by the water in said water tank such that the compressed gasin said compressed gas tank is in communication with the water in saidwater tank only when said first valve is opened.
 12. The passive safetysystem of claim 1 wherein said water tank is not pressurized unless saidfirst valve is opened during a nuclear shutdown.
 13. The passive safetysystem of claim 1 wherein said source of compressed gas is connected tosaid water tank through a pressure relief and expansion valve thatmaintains pressure in said water tank at or below a preset maximumpressure level when said first valve is open.
 14. The passive safetysystem of claim 1 further including a heat exchanger, wherein compressedgas from said source of compressed gas passes through said heatexchanger that extracts heat from said water expelled from said watertank by said compressed gas.
 15. A method for removing decay heat from anuclear reactor after shutdown, comprising: providing a tank of waterhaving an inlet and an outlet, providing an activatable source ofcompressed gas, providing a cooling fluid inlet connection to saidnuclear reactor, connecting said source of compressed gas to said inletconnection of said tank, connecting said outlet of said tank to saidcooling water input connection of said nuclear reactor, activating saidsource of compressed gas so that said gas enters said inlet of said tankand urges said water to leave said tank by said outlet and flow to saidnuclear reactor via said cooling water input connection, whereby whensaid source of compressed gas is activated, said water flows into saidnuclear reactor and removes said decay heat.
 16. The method of claim 15wherein said activatable source of compressed gas is connected to saidcooling fluid connection of said reactor, whereby when said activatablesource of compressed gas is activated, said compressed gas flows intosaid cooling fluid connection of said reactor and cools said reactor.17. The method of claim 15 wherein said tank and said source ofcompressed gas are buried underground.
 18. The method of claim 15wherein said tank and said activatable source of compressed gas areportable.
 19. The method of claim 15 wherein said tank and saidactivatable source of compressed gas are mounted on at least one movablewheeled conveyance.
 20. The passive safety system of claim 15 whereinsaid wheeled conveyance, said tank, and said source of compressed gasare concealed in a trench.
 21. The passive safety system of claim 15wherein said wheeled conveyance is at least one railroad car.
 22. Apassive safety system for cooling a nuclear power plant after shutdowncomprising: a nuclear power plant including a reactor pressure vessel, afirst turbine arranged to receive steam from said nuclear power plantvia a first conduit, said first turbine being connectable to a firstgenerator for generating electricity, reservoir means for storingcompressed air, a second plumbing connection between said reservoirmeans and said nuclear power plant, a first valve in said secondplumbing connection, a second turbine connectable to an auxiliarygenerator for generating electricity, a heat exchanger comprising aninterior volume and a coil within said interior volume, said coil havingfirst and second ends, a third plumbing connection between saidreservoir means and said interior volume of said heat exchanger, asecond valve in said second plumbing connection, a fourth plumbingconnection between said interior volume of said heat exchanger and saidsecond turbine, a fifth plumbing connection between said first turbineand said first end of said coil in said heat exchanger, a sixth plumbingconnection between said second end of said coil in said heat exchangerand said nuclear power plant, said fifth plumbing connection beingarranged to return hot water and steam from said first turbine to saidnuclear power plant via said coil in said heat exchanger, therebyheating said coil, whereby when said first valve is opened and saidsecond valve is closed, said compressed air passes through said secondplumbing connection into said nuclear power plant, thereby cooling saidnuclear power plant, and when said first valve is closed and said secondvalve is opened, said compressed air passes through said third plumbingconnection, through said interior volume of said heat exchanger so thatsaid compressed air is heated and thereby expanded by heat transfer fromsaid coil as said compressed air passes through said interior volume ofsaid heat exchanger, and then said compressed air passes through saidfourth plumbing connection and into said second turbine, therebyactivating said second turbine and said second generator and generatingelectricity.
 23. The passive safety system in claim 22, furtherincluding manifold surrounding said fuel rods so that when saidcompressed gas directed into said nuclear reactor during said shutdownsaid manifold will direct said compressed air around nuclear fuel rodsso that said compressed air expands around and in proximity to said fuelrods, thereby extracting decay heat from said fuel rods.
 24. The passivesafety system of claim 22 wherein said first valve in said first pipe isa flow control valve of the type selected from the group consisting ofmanually or automatically operable flow control valves.
 25. The passivesafety system of claim 24 wherein said first valve is in thermal contactwith said reactor pressure vessel.